Nicotinamide N-methyltransferase (NNMT) is one of the most abundant small molecule methyltransferases in the human body and is primarily responsible for the N-methylation of the nicotinamide (vitamin B3). Employing the cofactor S-adenosyl-l-methionine, NNMT transfers a methyl group to the pyridine nitrogen of nicotinamide to generate N-methylnicotinamide. Interestingly, NNMT is also able to N-methylate a variety of other pyridine-containing small molecules, suggesting a secondary role for the enzyme in the detoxification of xenobiotics. A number of recent studies have also revealed links between NNMT overexpression and a variety of diseases, including multiple cancers, Parkinson's disease, diabetes, and obesity. To facilitate further study of both the substrate scope and potential for inhibitor development, we here describe the development of a new NNMT activity assay. The assay makes use of ultra-high-performance hydrophilic interaction chromatography, allowing for rapid separation of the reaction products, coupled with quadrupole time-of-flight mass spectrometric detection, providing for enhanced sensitivity and enabling high-throughput sample analysis. We successfully demonstrated the general applicability of the method by performing kinetic analyses of NNMT-mediated methylation for a range of pyridine-based substrates. These findings also provide new insight into the diversity of substrate recognition by NNMT in a quantitative manner. In addition, we further established the suitability of the assay for the identification and characterization of small molecule inhibitors of NNMT. To do so, we investigated the inhibition of NNMT by the nonspecific methyltransferase inhibitors sinefungin and S-adenosyl-l-homocysteine, revealing IC50 values in the low micromolar range. The results of these inhibition studies are particularly noteworthy as they will permit future efforts toward the development of new NNMT-specific inhibitors.